What does it do and do I need it. CPI will parse a picture file (jpg, bmp, gif) to gather information from the picture ie width & height, from this it works out what is the maxium size your image will print at, at a given dpi plus the sort of file size that is needed. Do you need CPI it's up to you! If you supply images to magazines you will want to know what is the largest size your image can be printed at without degredation to the image. Most magazine reprographic departments would like you to supply your images at the maxium size and dpi that they use. So if CPI says your image can go to say 23cm wide @ 300 dpi you should open your image and resize it to this have a look and then save.